Document WO 01/48531 discloses a display panel structure (see especially FIGS. 49 to 55) in the form of an array of cells each of which comprises two deformable dielectric layers, which meet at a common interface. One of said dielectrics can be air; the other is preferably polymer material acting as a relief forming gel. For each cell a first electrode structure, a support electrode structure is arranged on one side (below) of said dielectric layers and a second electrode structure, a signal electrode structure on the other side (above) of said layers, there being means for providing signals to the signal electrodes in order to create electric fields through the two dielectric layers and to further create surface reliefs on the gel surface at the interface of said two dielectrics. The periodical, sinusoidally varying reliefs created on the gel surface in each of the cells allows under the effect of light from a light source to create images on the display panel, which are viewable by the naked eye.
FIG. 1 illustrates schematically the general principle of physics, which can be observed in connection with dielectric substances and which is also utilized in the aforementioned optical display device. Dielectric substance can be defined as a substance in which an electric field may be maintained with zero or near zero power dissipation, that is the electrical conductivity of the substance is zero or near zero. An electric field E going through an interface where dielectricity changes, such as the interface between air and polymer, causes a force F onto the surface of the dielectric having the higher dielectricity constant. This ponderomotive force F is proportional to the square of the electric field E at that point. In the case of an interface between air and polymer, the ponderomotive force F acts onto the polymer surface into the direction of the air.
The electric field generated between a support electrode and a signal electrode and penetrating through the interface of the two dielectrics within a light modulator cell is inhomogeneous over the polymer surface; the electric field is strongest directly under a signal electrode and weakest in the middle between two adjacent signal electrodes. This situation is valid for any practical dimensions of the light modulator cells and corresponding devices. Therefore, the force acting onto the polymer surface is inhomogeneous as well and a deformation of the polymer surface causing the surface to deviate from a flat plane can be observed. An optimum situation would be such that maximum ponderomotive force is reached directly under the center part of a signal electrode and correspondingly no force is applied in the middle between two adjacent signal electrodes. This would provide maximum height for the surface relief, because it allows the maximum flow of the polymer material towards the area where the electric field is strongest. The concept of polymer flow is essential here, since most polymers are substantially uncompressible and therefore cannot simply expand in volume under the effect of an electric field.
FIG. 2 clarifies the aforementioned prior art solution by showing two adjacent pixels (cells) L,R in a possible pixel line of a display panel or corresponding light modulator device. Each pixel comprises several strip like signal electrodes 10 arranged parallel respect to each other on a substrate 11, which may be for example glass. FIG. 2 shows an end view of the signal electrodes 10 thus revealing only the width and mutual distance between the electrodes 10. A dielectric and viscoelastic gel material 12, for example polymer, is applied onto a support electrode 14 and facing the signal electrodes 10. A gap 13, for example an air gap, is left between the surface of the viscoelastic layer 12 and the signal electrodes 10. The usually transparent support electrode 14 providing AC or DC potential is preferably made of indium tin oxide (ITO), as is known in the art, on the surface of a transparent front plate 15. The front plate 15 may be for example glass.
The light may enter the structure depicted in FIG. 2 either through the front plate 15 or through the substrate 11. The device can be designed to operate either in transmissive or reflective mode depending on the light transmitting or reflecting properties of the various elements of the device.
In FIG. 2 the left pixel L is switched on by providing suitable voltage to the respective signal electrodes 10, whereas the right pixel R is switched off, that is no signal is fed to the signal electrodes 10 belonging to said pixel R. Arrows 16 marked for the left pixel L illustrate schematically the ponderomotive forces acting on the surface of the gel layer 12. Through proper selection of the signal electrode 10 dimensions (width) and mutual positioning (distance between adjacent and parallel electrodes) a sinusoidal surface structure, a surface relief can be created on the polymer gel 12 surface. Illuminating light onto the sinusoidal gel 12 surface in pixel L through the front plate 15 or substrate 11 refracts light into a slightly different direction than illuminating a plane gel surface in pixel R. By this means and using an optical blocking filter for the pixel in off state, a pixel can be electrically switched on and off.
The major shortcomings of the light modulators, like the display panels described in WO 01/48531, which are based on electrically controlled deformation of dielectric and viscoelastic materials may be associated with the practical difficulties in producing desired profiles for the gel reliefs. This further impairs the light modulating capabilities of the devices, for example the capabilities of individual pixels or cells.
With the goal of bringing display quality closer to that of a paper print, for example brightness and contrast of the displays must be further improved. In order to allow the use of display devices in small-size portable devices, the power consumption and thickness of the display devices should also be further reduced. To make mass production of portable devices possible, the manufacturing technology of the display devices should be simple in order to allow lower prices. The displays should also be fast enough to be able to display video or similar fast changing image content without degration of the image quality. The aforementioned requirements also apply on other light modulator devices than display panels. Other applications which may be based on the use of electrically controlled deformation of dielectric and viscoelastic transparent materials include, but are not limited to, electrically controlled diffractive or refractive lenses, or electrically reconfigurable optical waveguide couplers.